JUSTIN SKOBLE AND DAN PORTNOY / COURTESY: NATIONAL SCIENCE FOUNDATION
L. monocytogenes, a rod-shaped bacterium, is a carrier of listeriosis, a general name given to the group of disorders caused by the organism, such as meningitis and encephalitis.

Tiny bacteria loom large over processes of life

FACTS OF THE MATTER
Richard Brill

THE DUTCH scientist Anton van Leeuwenhoek was the first to observe bacteria in 1674 using a single-lens microscope of his own design. He called them "animalcules" and published his observations in a series of letters to the Royal Society. The name bacterium was introduced much later, in 1828.

Ferdinand Cohn (1828-1898) is usually credited with founding the science of bacteriology, later a subdiscipline of microbiology. Cohn was a botanist whose studies on algae and photosynthetic bacteria led him to describe several bacteria and formulate a scheme for their taxonomic classification.

Bacteria are microscopically tiny, one-celled, sometimes colonial organisms lacking chlorophyll. They are prokaryotic organisms that reproduce asexually through cell division and live in virtually all environments. They are shaped like spheres, rods or spirals and usually have cell walls but lack a structurally discrete nucleus surrounded by a membrane.

The DNA of prokaryotic cells is strung out in the cytoplasm of the cells, in contrast with eucaryotic cells in which DNA is coiled within the nucleus, confined by a nuclear membrane and containing smaller cell-like structures called organelles.

Prokaryotes are the most primitive type of cell and represent the closest living relatives of the earliest life on planet Earth, some having changed little in 3 billion years.

The much more complicated eucaryotic cells that form multicelled plants and animals reproduce sexually, with one-half of the genetic material coming from each of two different cells. Eucaryotics do not appear in the fossil record until a billion years after the prokaryotes.

AS MUCH as 90 percent of the cells in our bodies are bacteria, which means that there are approximately 10 times as many bacterial cells as human cells in the human body.

There are various estimates of the total number of cells in the human body, but most are in the range of 10 to the 14th power, or 100 trillion.

That's 14 zeroes if you're scoring at home.

Unfathomably large numbers of bacteria are on the skin and in the digestive tract. The vast majority are rendered harmless by the protective effects of the immune system, and many are beneficial, if not essential, for maintaining good health.

Escherichia coli (E. coli) is one of the main species of bacteria living in the lower intestines of mammals, known as gut flora. They are abundant in humans; the number of individual E. coli cells excreted every day averages between 100 billion and 10 trillion.

Sarkis K. Mazmanian of Caltech writes, "Immunologic imbalances underlie many human diseases. Protection from autoimmune disorders, resistance to infections and the control of cancers require the proper functioning of the immune system. Fortunately, our immune system is not alone in this struggle. The human body represents a scaffold upon which multitudes of commensal species build residence, creating a diverse ecosystem with members of five of the six kingdoms of life."

Many other species of bacteria cause infectious diseases, including tuberculosis, cholera, syphilis, anthrax, leprosy and bubonic plague.

Tuberculosis alone kills 2 million people a year. In regions where antibiotics are used to treat bacterial infections, especially in hospitals, and in various agricultural processes antibiotic resistance to many bacteria is becoming common.

In industry, bacteria are important in processes such as waste-water treatment, the production of cheese and yogurt, and the manufacture of antibiotics and other chemicals.

Genetically engineered E. coli bacteria are used industrially to manufacture human proteins such as insulin much more cheaply than past methods, making the essential insulin available to a larger number of diabetics, many of whom would not survive without it.

BACTERIA ALSO play a starring but often taken-for-granted role in ecosystems. An astonishing variety of different bacteria are responsible for the decay of organic material and nitrogen fixation in soil, performing the essential role of recycling nutrients through the ecosystem quickly.

Biologist Noah Fierer of the University of Colorado said, "We step on soil every day, but few people realize that 'dirt' supports a complex community of microorganisms that plays a critical role on Earth. The number of bacterial species in a spoonful of soil is likely to exceed the total number of plant species in all of the United States."

Bacteria in the soil are essential for providing nutrients as they consume and decompose simple carbon compounds, such as secretions from roots and fresh plant litter.

The decomposition converts the energy and materials stored in the soil as organic matter into forms that are useful to the rest of an intricate yet diverse soil food web.

A layer of soil can contain millions of different species of bacteria, about one ton of bacteria per acre of soil. No one really knows how many different species there are, and estimating it is not easy. Less than 1 percent of the bacteria present in soil will tolerate the culturing and subsequent microscopic observation that is required to identify and classify them.

Other species of soil bacteria produce substances that help bind soil particles into small aggregates, which affects water movement when the particles have diameters in the range of one ten-thousandth to one one-hundredth of an inch. These fine aggregates act to filter particulate matter out of ground water and render it safe for drinking.

Unfortunately, bacteria do not significantly filter chemicals out of natural water supplies, especially the millions of tons of synthetic chemicals we flush away on a daily basis.

Bacteria also appear to have an important role in evolution, supplying us with genes. It's long been a mystery why the speed and complexity of evolution appear to increase with time, and how such complex creatures as ourselves managed to evolve at all.

In the fossil record, single-celled life first appeared in rock about 3.5 billion years old. After that, it took about 2.5 billion more years for multicellular life to evolve and another half-billion for complex life to appear. The diverse collection of plants, animals, insects, birds and other species that populate the earth evolved from those simplest multicellular forms in the short half-billion years since then.

New studies by scientists at Rice University suggest the explanation is that bacteria and viruses constantly exchange transposable chunks of DNA between species, thus making it possible for life forms to evolve faster than if they relied entirely on sexual selection or random genetic mutations.

"We have developed the first exact solution of a mathematical model of evolution that accounts for this cross-species genetic exchange," said Michael Deem, the John W. Cox Professor in Biochemical and Genetic Engineering and professor of physics and astronomy.

The science has grown unimaginably beyond what its founders could have envisioned, and in the next half-century will likely grow equally beyond what we can imagine today.

Richard Brill, professor of science at Honolulu Community College, teaches earth and physical science and investigates life and the universe. E-mail questions and comments to rickb@hcc.hawaii.edu